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  • richardmitnick 8:30 pm on February 21, 2019 Permalink | Reply
    Tags: , , , , Molecular ensemble, , Photocatalysis, , PtPOP, , , ,   

    From SLAC National Accelerator Lab: “Researchers watch molecules in a light-triggered catalyst ring ‘like an ensemble of bells’’ 

    From SLAC National Accelerator Lab

    February 21, 2019
    Ali Sundermier

    1
    Synchronized molecules
    When photocatalyst molecules absorb light, they start vibrating in a coordinated way, like an ensemble of bells. Capturing this response is a critical step towards understanding how to design molecules for the efficient transformation of light energy to high-value chemicals. (Gregory Stewart/SLAC National Accelerator Laboratory)

    A better understanding of these systems will aid in developing next-generation energy technologies.

    Photocatalysts ­– materials that trigger chemical reactions when hit by light – are important in a number of natural and industrial processes, from producing hydrogen for fuel to enabling photosynthesis.

    Now an international team has used an X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory to get an incredibly detailed look at what happens to the structure of a model photocatalyst when it absorbs light.

    The researchers used extremely fast laser pulses to watch the structure change and see the molecules vibrating, ringing “like an ensemble of bells,” says lead author Kristoffer Haldrup, a senior scientist at Technical University of Denmark (DTU). This study paves the way for deeper investigation into these processes, which could help in the design of better catalysts for splitting water into hydrogen and oxygen for next-generation energy technologies.

    “If we can understand such processes, then we can apply that understanding to developing molecular systems that do tricks like that with very high efficiency,” Haldrup says.

    The results published last week in Physical Review Letters.

    Molecular ensemble

    The platinum-based photocatalyst they studied, called PtPOP, is one of a class of molecules that scissors hydrogen atoms off various hydrocarbon molecules when hit by light, Haldrup says: “It’s a testbed – a playground, if you will – for studying photocatalysis as it happens.”

    At SLAC’S X-ray laser, the Linac Coherent Light Source (LCLS), the researchers used an optical laser to excite the platinum-containing molecules and then used X-rays to see how these molecules changed their structure after absorbing the visible photons.

    SLAC/LCLS

    The extremely short X-ray laser pulses allowed them to watch the structure change, Haldrup says.

    The researchers used a trick to selectively “freeze” some of the molecules in their vibrational motion, and then used the ultrashort X-ray pulses to capture how the entire ensemble of molecules evolved in time after being hit with light. By taking these images at different times they can stitch together the individual frames like a stop-motion movie. This provided them with detailed information about molecules that were not hit by the laser light, offering insight into the ultrafast changes occurring in the molecules when they are at their lowest energy.

    Swimming in harmony

    Even before the light hits the molecules, they are all vibrating but out of sync with one another. Kelly Gaffney, co-author on this paper and director of SLAC’s Stanford Synchrotron Radiation Lightsource, likens this motion to swimmers in a pool, furiously treading water.

    SLAC SSRL Campus


    SLAC/SSRL


    SLAC/SSRL

    When the optical laser hits them, some of the molecules affected by the light begin moving in unison and with greater intensity, switching from that discordant tread to synchronized strokes. Although this phenomenon has been seen before, until now it was difficult to quantify.

    “This research clearly demonstrates the ability of X-rays to quantify how excitation changes the molecules,” Gaffney says. “We can not only say that it’s excited vibrationally, but we can also quantify it and say which atoms are moving and by how much.”

    Predictive chemistry

    To follow up on this study, the researchers are investigating how the structures of PtPOP molecules change when they take part in chemical reactions. They also hope to use the information they gained in this study to directly study how chemical bonds are made and broken in similar molecular systems.

    “We get to investigate the very basics of photochemistry, namely how exciting the electrons in the system leads to some very specific changes in the overall molecular structure,” says Tim Brandt van Driel, a co-author from DTU who is now a scientist at LCLS. “This allows us to study how energy is being stored and released, which is important for understanding processes that are also at the heart of photosynthesis and the visual system.”

    A better understanding of these processes could be key to designing better materials and systems with useful functions.

    “A lot of chemical understanding is rationalized after the fact. It’s not predictive at all,” Gaffney says. “You see it and then you explain why it happened. We’re trying to move the design of useful chemical materials into a more predictive space, and that requires accurate detailed knowledge of what happens in the materials that already work.”

    LCLS and SSRL are DOE Office of Science user facilities. This research was supported by DANSCATT; the Independent Research Fund Denmark; the Icelandic Research Fund; the Villum Foundation; and the AMOS program within the Chemical Sciences, Geosciences and Biosciences Division of the DOE Office of Basic Energy Sciences.

    See the full article here .


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    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

     
  • richardmitnick 9:28 am on November 5, 2018 Permalink | Reply
    Tags: , MOFs-Metallic Organic Frameworks, New material cleans and splits water, Organic pollutant degradation, Photocatalysis   

    From École Polytechnique Fédérale de Lausanne: “New material cleans and splits water” 

    EPFL bloc

    From École Polytechnique Fédérale de Lausanne

    05.11.18
    Nik Papageorgiou

    1
    Simultaneous photocatalytic hydrogen generation and dye degradation using a visible light active metal–organic framework. Creidt: Alina-Stavroula Kampouri/EPFL

    Researchers at EPFL’s Institute of Chemical Sciences and Engineering have developed a photocatalytic system based on a material in the class of metal-organic frameworks. The system can be used to degrade pollutants present in water while simultaneously producing hydrogen that can be captured and used further.

    Some of the most useful and versatile materials today are the metal-organic frameworks (MOFs). MOFs are a class of materials demonstrating structural versatility, high porosity, fascinating optical and electronic properties, all of which makes them promising candidates for a variety of applications, including gas capture and separation, sensors, and photocatalysis.

    Because MOFs are so versatile in both their structural design and usefulness, material scientists are currently testing them in a number of chemical applications. One of these is photocatalysis, a process where a light-sensitive material is excited with light. The absorbed excess energy dislocates electrons from their atomic orbits, leaving behind “electron holes”. The generation of such electron-hole pairs is a crucial process in any light-dependent energy process, and, in this case, it allows the MOF to affect a variety of chemical reactions.

    A team of scientists at EPFL Sion led by Kyriakos Stylianou at the Laboratory of Molecular Simulation, have now developed a MOF-based system that can perform not one, but two types of photocatalysis simultaneously: production of hydrogen, and cleaning pollutants out of water. The material contains the abundantly available and cheap nickel phosphide (Ni2P), and was found to carry out efficient photocatalysis under visible light, which accounts to 44% of the solar spectrum.

    The first type of photocatalysis, hydrogen production, involves a reaction called “water-splitting”. Like the name suggests, the reaction divides water molecules into their constituents: hydrogen and oxygen. One of the bigger applications here is to use the hydrogen for fuel cells, which are energy-supply devices used in a variety of technologies today, including satellites and space shuttles.

    The second type of photocatalysis is referred to as “organic pollutant degradation”, which refers to processes breaking down pollutants present in water. The scientists investigated this innovative MOF-based photocatalytic system towards the degradation of the toxic dye rhodamine B, commonly used to simulate organic pollutants.

    The scientists performed both tests in sequence, showing that the MOF-based photocatalytic system was able to integrate the photocatalytic generation of hydrogen with the degradation of rhodamine B in a single process. This means that it is now possible to use this photocatalytic system to both clean pollutants out of water, while simultaneously producing hydrogen that can be used as a fuel.

    “This noble-metal free photocatalytic system brings the field of photocatalysis a step closer to practical ‘solar-driven’ applications and showcases the great potential of MOFs in this field,” says Kyriakos Stylianou.

    Other contributors

    University College London

    Science paper:
    Concurrent Photocatalytic Hydrogen Generation and Dye Degradation Using MIL‐125‐NH2 under Visible Light Irradiation
    Advanced Functional Materials

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    EPFL campus

    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

     
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